Serious non-infectious chronic illnesses relating to diet and lifestyle are major causes of morbidity and mortality in affluent industrialised countries and also in emerging ones with greater affluence. These include coronary heart disease, diverticular disease, certain cancers (especially of the colon and rectum) and diabetes. Cereal foods have significant potential to improve human health through lowering the risk of these conditions. The benefits may be obtained through consumption of processed foods containing whole grains or their constituents including complex carbohydrates—starch and non-starch polysaccharides (NSP, major components of dietary fibre). NSP are resistant to digestion by human small intestinal enzymes which helps to explain their effectiveness in increasing faecal bulk and relieving constipation (Topping & Clifton, 2001). While starch can be digested (theoretically to completion) in the human small intestine, some escapes into the large bowel. This fraction is resistant starch (RS) which, together with a variable fraction of NSP, is metabolised by the large bowel microflora (Topping & Clifton, 2001). Short chain fatty acids (SCFA) are major end products of this fermentation and they promote important aspects of large bowel function-stimulation of fluid and electrolyte absorption, modulation of muscular contraction and visceral perfusion (Topping & Clifton, 2001). One of the principal SCFA, butyrate, may also play a role in promoting a normal phenotype in colonocytes, and enhancing normally controlled colonocyte proliferation and lowering the risk of colo-rectal cancer. The latter malignancy is a substantial cause of early morbidity in affluent industrialised countries. A further consequence of slower starch small intestinal digestibility is the potential to lower the rate of entry of glucose into the circulation and, thus, a lesser demand for insulin. This is measured as glycaemic index (GI) which is emerging as a substantial factor in disease risk.
It is emerging also that many of the actions ascribed to dietary fibre may actually be due to RS (Topping & Clifton, 2001). RS intakes are low in populations at high risk of the diseases of affluence and modification of convenience foods to enhance the content and action of RS is considered to be an effective means of improving nutrition for public health at the population level. This may be put into practice through encouraging the consumption of specific foods, such as beans or wholegrain (brown) rice, which are intrinsically high in RS. Another approach is to enrich convenience foods with RS as an added ingredient.
Wheat is a staple food in many countries and supplies approximately 20% of the food kilojoules for the total world population. The processing characteristics of wheat make it the preferred base for most cereal-based processed products such as bread, pasta and noodles. Wheat consumption is increasing world-wide with increasing affluence. Breadwheat (Triticum aestivum) is a hexaploid having three different genomes, A, B and D, and most of the known genes in wheat are present in triplicate, one on each genome. The hexaploid nature of the breadwheat genome makes finding and combining gene mutations in each of the three genomes a challenge. The presence of three genomes has a buffering effect by masking mutations in individual genomes, in contrast to the more readily identified mutations in diploid species. Known variation in wheat starch structure has been limited relative to the variation available in maize or rice. Another contributing factor to this is that the transformation efficiency of wheat has lagged behind that for other cereals.
The synthesis of starch in the endosperm of higher plants is carried out by a suite of enzymes that catalyse four key steps. Firstly, ADP-glucose pyrophosphorylase activates the monomer precursor of starch through the synthesis of ADP-glucose from G-1-P and ATP. Secondly, the activated glucosyl donor, ADP-glucose, is transferred to the non-reducing end of a pre-existing α1-4 linkage by starch synthases. Thirdly, starch branching enzymes introduce branch points through the cleavage of a region of α-1,4 linked glucan followed by transfer of the cleaved chain to an acceptor chain, forming a new α-1,6 linkage. Starch branching enzymes are the only enzymes that can introduce the α-1,6 linkages into α-polyglucans and therefore play an essential role in the formation of amylopectin. Finally, starch debranching enzymes remove some of the branch linkages although the mechanism through which they act is unresolved (Myers et al., 2000).
While it is clear that at least these four activities are required for normal starch granule synthesis in higher plants, multiple isoforms of each of the four activities are found in the endosperm of higher plants and specific roles have been proposed for individual isoforms on the basis of mutational analysis (Wang et al, 1998, Buleon et al., 1998) or through the modification of gene expression levels using transgenic approaches (Abel et al., 1996, Jobling et al., 1999, Scwall et al., 2000). However, the precise contributions of each isoform of each activity to starch biosynthesis are still not known, and it is not known whether these contributions differ markedly between species. In the cereal endosperm, two isoforms of ADP-glucose pyrophosphorylase are present, one form within the amyloplast, and one form in the cytoplasm (Denyer et al., 1996, Thorbjornsen et al., 1996). Each form is composed of two subunit types. The shrunken (sh2) and brittle (bt2) mutants in maize represent lesions in large and small subunits respectively (Giroux and Hannah, 1994). Four classes of starch synthase are found in the cereal endosperm, an isoform exclusively localised within the starch granule, granule-bound starch synthase (GBSS), two forms that are partitioned between the granule and the soluble fraction (SSI, Li et al., 1999a, SSII, Li et al., 1999b) and a fourth form that is entirely located in the soluble fraction, SSIII (Cao et al, 2000, Li et al., 1999b, Li et al, 2000). GBSS has been shown to be essential for amylose synthesis (Shure et al., 1983), and mutations in SSII and SSIII have been shown to alter amylopectin structure (Gao et al, 1998, Craig et al., 1998). No mutations defining a role for SSI activity have been described.
Three forms of branching enzyme are expressed in the cereal endosperm, branching enzyme I (SBEI), branching enzyme IIa (SBEIIa) and branching enzyme IIb (SBEIIb) (Hedman and Boyer, 1982, Boyer and Preiss, 1978, Mizuno et al., 1992, Sun et al., 1997). Genomic and cDNA sequences have been characterized for rice (Nakamura and Yamanouchi, 1992), maize (Baba et al., 1991; Fisher et al., 1993; Gao et al., 1997) and wheat (Repellin et al., 1997; Nair et al., 1997; Rahman et al., 1997). Sequence alignment reveals a high degree of sequence similarity at both the nucleotide and amino acid levels and allows the grouping into the SBEI, SBEIIa and SBEIIb classes. SBEIIa and SBEIIb generally exhibit around 80% sequence identity to each other, particularly in the central regions of the genes. SBEIIa and SBEIIb may also be distinguished by their expression patterns. SBEIIb in maize is specifically expressed in endosperm while SBEIIa is present in every tissue of the plant.
In wheat endosperm, SBEI (Morell et al, 1997) is found exclusively in the soluble fraction, while SBEIIa and SBEIIb are found in both soluble and starch-granule associated fractions (Rahman et al., 1995). In maize and rice, high amylose phenotypes have been shown to result from lesions in the SBEIIb gene, also known as the amylose extender (ae) gene (Boyer and Preiss, 1981, Mizuno et al., 1993; Nishi et al., 2001). In these SBEIIb mutants, endosperm starch grains showed an abnormal morphology, amylose content was significantly elevated, the branch frequency of the residual amylopectin was reduced and the proportion of short chains (<DP17, especially DP8-12) was lower. Moreover, the gelatinisation temperature of the starch was increased. In addition, there was a significant pool of material that was defined as “intermediate” between amylose and amylopectin (Boyer et al., 1980, Takeda, et al., 1993b). In contrast, maize plants mutant in the SBEIIa gene due to a mutator (Mu) insertional element and consequently lacking in SBEIIa protein expression were indistinguishable from wild-type plants in the branching of endosperm starch (Blauth et al., 2001), although they were altered in leaf starch. Similarly, rice plants deficient in SBEIIa activity exhibited no significant change in the amylopectin chain profile in endosperm (Nakamura 2002). In both maize and rice, the SBEIIa and SBEIIb genes are not linked in the genome.
Mutations in wheat SBEIIa or SBEIIb or the phenotypes of wheat lines carrying these mutations have not been reported. Known mutants in wheat are for the waxy gene (GBSS, Zhao and Sharp, 1998) and a mutant entirely lacking the SGP-1 protein (Yamamori et al, 2000) which was produced by crossing lines which were lacking the A, B and D genome specific forms of SGP-1 (SSII) protein as assayed by protein electrophoresis. Examination of the SSII null seeds showed that the mutation resulted in alterations in amylopectin structure, deformed starch granules, and an elevated relative amylose content to about 30-37% of the starch, which was an increase of about 8% over the wild-type level (Yamamori et al., 2000). Amylose was measured by colorimetric measurement, amperometric titration (both for iodine binding) and a concanavalin A method. Starch from the SSII null mutant exhibited a decreased gelatinisation temperature compared to starch from an equivalent, non-mutant plant. Starch content was reduced from 60% in the wild-type to below 50% in the SSII-null grain.
In maize, the dull1 mutation causes decreased starch content and increased amylose levels in endosperm, with the extent of the change depended on the genetic background, and increased degree of branching in the remaining amylopectin (Shannon and Garwood, 1984). The gene corresponding to the mutation was identified and isolated by a transposon-tagging strategy using the transposon mutator (Mu) and shown to encode the enzyme designated starch synthase II (SSII) (Gao et al., 1998). The enzyme is now recognized as a member of the SSIII family in cereals (Li et al., 2003). Mutant endosperm had reduced levels of SBEIIa activity associated with the dull1 mutation. No corresponding mutation has been reported in other cereals. It is not known if these findings are relevant to other cereals, for example wheat.
Two types of debranching enzymes are present in higher plants and are defined on the basis of their substrate specificities, isoamylase type debranching enzymes, and pullulanase type debranching enzymes (Myers et al., 2000). Sugary-1 mutations in maize and rice are associated with deficiency of both debranching enzymes (James et al., 1995, Kubo et al., 1999) however the causal mutation maps to the same location as the isoamylase-type debranching enzyme gene. Representative starch branching enzyme sequences from genes that have been cloned from cereals are listed in Table 1.
TABLE 1Starch branching enzyme genes characterized from cereals.Type ofSpeciesSBE isoformcloneAccession No.ReferenceWheatSBEIcDNA andAJ237897 SBEI gene)Baga et al., 1999genomicAF002821 (SBEI pseudogeneRahman et al., 1997,AF076680 (SBEI gene)Rahman et al., 1999AF076679 (SBEI cDNA)SBEIcDNAY12320Repellin et al., 1997SBEIIacDNAY11282Nair et al., 1997SBEIIacDNA andAF338432 (cDNA)Rahman et al., 2001genomicAF338431 (gene)SBEIIbcDNA andWO 01/62934genomicSBEIIbcDNAWO 00/15810RiceSBEIcDNAD10752Nakamura andYamanouchi, 1992SBEIgenomicD10838Kawasaki et al., 1993RBE3cDNAD16201Mizuno et al., 1993BarleySBEIIa andcDNA andAF064563 (SBEIIb gene)Sun et al., 1998SBEIIbgenomicAF064561 (SBEIIb cDNA)AF064562 (SBEIIa gene)AF064560 (SBEIIa cDNA)MaizeSBEIcDNAU17897Fisher et al., 1996genomicAF072724Kim et al., 1998aSBEIIbcDNAL08065Fisher et al., 1993genomicAF072725Kim et al., 1998SBEIIacDNAU65948Gao et al., 1997
Starch composition, in particular the form called resistant starch which may be associated with high amylose content, has important implications for bowel health, in particular health of the large bowel. The beneficial effects of resistant starch are thought to result from the provision of a nutrient to the large bowel wherein the intestinal microflora are given an energy source which is fermented to form inter alia short chain fatty acids. These short chain fatty acids provide nutrients for the colonocytes, enhance the uptake of certain nutrients across the large bowel and promote physiological activity of the colon. Generally if resistant starches or other dietary fibre are not provided the colon is metabolically relatively inactive.
Whilst chemically or otherwise modified starches can be utilised in foods that provide functionality not normally afforded by unmodified sources, such processing has a tendency to either alter other components of value or carry the perception of being undesirable due to processes involved in modification. Therefore it is preferable to provide sources of constituents that can be used in unmodified form in foods. Although high amylose maize and barley varieties are known, products from these cereals have disadvantages compared to a very high amylose wheat for products where wheat is the preferred cereal, for example in bread, pasta or noodles. There is therefore an opportunity for a large scale improvement in public health including bowel health and metabolic health through the alteration of wheat starch, which may provide an increase in resistant starch and reduction in glycemic index when provided in the diet.
On passage from the ileum, resistant starches are metabolised by the anaerobic microflora of the caecum and colon which produce the enzymes necessary for polysaccharide hydrolysis and catabolism. Breakdown is effected by bacterial species very similar to those found in the rumen of obligate herbivores and with very similar products: gases, such as carbon dioxide, methane and hydrogen, and short chain fatty acids (SCFA). The principle SCFA formed are acetate, propionate and butyrate in the rough molar proportions 60:20:20. These three acids contribute some 80-90% of total colonic SCFA, the remainder being branched chain and other fatty acids formed from the breakdown of dietary and endogenous protein. Animals fed resistant starch have shown higher colonic SCFA and in some cases increased bacterial mass in the colon. Many of the effects of resistant starch in the colon are probably mediated through SCFA.
The role of dietary fibre in the prevention and management of simple constipation is beyond question. Fibres vary in their effects on bowel function. Cereal brans such as wheat and rice brans that are high in insoluble NSP appear to be most effective in easing problems of Taxation through shortening transit time, softening stools through raised water holding, increasing stool volume and weight in the form of bacteria and undigested and non-fermentable material.
Although it is convenient to explain the actions of fibre-rich foods such as wheat bran solely in terms of stool mass, this is not quite correct. However, the increase in faecal bulk in humans eating mixed diets is considerably higher than predicted from their non starch polysaccharide content—the “carbohydrate gap” (Stephen (1991) Can J Physiol. Pharmacol. 69:116-20). Starch is thought to fill this gap and contribute to the greater faecal bulk through bacterial proliferation, by providing a fermentation substrate, (both glucose, and certain SCFA) as well as providing physical bulk.
Increases in microbial mass from undigestible carbohydrate fermentation contributes directly to stool bulk, which is a large part of the stool weight. Bacteria are about 80% water and have the ability to resist dehydration, as such they contribute to water-holding in fecal material. The number of bacteria in human feces is approximately 4×1011-8×1011/g dry feces, and makes up to about 50% of fecal solids in subjects on a Western diet. Gas production from colonic fermentation can also have some influence on stool bulk. Trapping of gas can contribute to increased volume and a decrease in fecal transit time.
The metabolic end products of fermentation, namely the gases, SCFA and increased microflora play a pivotal role in the physiological effects of the undigestible carbohydrate in the colon and implications for local effects in the colon and systemic effects. The gases produced from fermentation by strict anaerobic species such as bacteriodes, some non-pathogenic species of clostridia and yeasts, anaerobic cocci and some species of lactobacilli are mostly released as flatulence or are absorbed and subsequently lost from the body through the lungs. However, some of the hydrogen and carbon dioxide produced from these microflora may be further metabolized to methane (CH4) by methanogenic bacteria, thus reducing intestinal gas pressure. Of these anaerobic microorganisms, the clostridia, eubacteria and anaerobic cocci are the most gas producing, while the bifidobacteria are the only group of common gut microflora that do not produce any gases.
Because resistant starch is not digested or absorbed, it also serves as a prebiotic for beneficial bacteria, such as bifidobacteria and lactobacilli. Multiplying beneficial bacteria reduce the pH level in the colon, making the environment uninhabitable for potentially harmful bacteria such as E. coli, clostridia, Veillonella and Klebsiella. The proliferation of beneficial bacteria provides significant health effects, including enhanced digestion and improved lactose intolerance, promoting the recycling of compounds such as estrogen, synthesizing vitamins, especially B-group vitamins, producing immune-stimulating compounds, inhibiting the growth of harmful bacteria, reducing the production of toxins and carcinogens, restoring normal intestinal bacteria during antibiotic therapy, and reducing the potential for several pathologies commonly associated with higher numbers of pathogenic intestinal bacteria. These include autoimmune illnesses such as ankylosing spondylitis and rheumatoid arthritis, certain cancers, yeast overgrowth, vaginitis, urinary tract infections, cirrhosis of the liver, food poisoning, antibiotic-associated diarrhea, inflammatory bowel diseases such as ulcerative colitis and Crohn's disease, necrotizing entercolitis and ileocecitis, food allergy and intolerance, intestinal gas and bloating, and irritable bowel syndrome.
The primary SCFA generated by fermentation are acetate, propionate and butyrate, accounting for 83-95% of the total SCFA concentration in the large intestine, which ranges from about 60-150 μmol/L. The concentrations of these acids are highest where concentrations of microflora are also highest, namely in the cecum and right or transverse colon. Corresponding to these higher acid levels, the pH is also typically lowest in the transverse colon (5.4-5.9) and gradually increases through the distal colon to 6.6-6.9. As the pH is reduced, the colonic environment becomes less favorable for toxin-producing and ill-health promoting microflora, such as E. coli, clostridia, and certain yeasts.
The pH range of digesta in the human colon needs to be established but in pigs on high fibre diets it ranges from approximately 6 in the proximal colon to >7 in the distal colon. The pKa of short chain fatty acids is <4.8 so that in the colon they are present largely as anions. SCFA are absorbed in the non-ionic form and are then ionized at intracellular pH to H+ and SCFA which are then exchanged for luminal Na+ and Cl− respectively. Some of the SCFA are also metabolized to HCO3 which is also exchanged for chloride ions. Therefore SCFA is beneficial in facilitating transporting ions that play an important role in metabolism.
Thus SCFA do not contribute to osmotic load to any great extent and may ameliorate diarrhoea through removal of sodium and water from the colonic lumen. However, because SCFA are present largely as anions, their absorption is relatively slow. For this reason and their presence in faeces, SCFA have been assumed to cause diarrhoea. That view is no longer held and diarrhoea is thought to occur only when the osmotic pressure of simple and complex carbohydrates in the colon raises the fluid volume excessively and bacteria cannot break down the carbohydrate sufficiently rapidly. In fact SCFA may have longer term preventative effects by stimulating growth of colonocytes thereby increasing the capacity of the colon.
Epidemiological data have shown that the level of dietary fibre is inversely related to incidence of bowel cancer and a meta-analysis of a large number of studies showed that fibre was protective in over 50%. It is not possible to discriminate the type of NSP or foods that were effective. A study by Cassidy et al (British Journal of Cancer 69; 937-942 (1994)) has shown that starch plays a protective role.
The role of fibre in the maintenance of colonic mucosal integrity is understood imperfectly. Experiments with animal models such as pigs have shown that the weight and thickness of the colon is increased with diets high in fibre—consistent with greater cell growth. The effect is not confined to fibre as Goodlad and Mathers ((1990) Brit J Nutr. 64; 569-587) have obtained similar increases in the hindgut of rats fed diets high in resistant starch. Other studies with rats have shown that the increase is probably not due to increased mass of digesta since an inert faecal bulking agent (kaolin) did not stimulate mucosal proliferation. In the same experiments it was shown that colonic infusion of short chain fatty acids enhanced colonocyte proliferation suggesting that they were the trophic agents (Sakata J. Nutr Sci Vitaminol 1986; 32: 355-362). It is likely that only propionate and butyrate are involved in these effects. Propionate is known to enhance colonic motility possibly through stimulating blood flow (Kvietis and Granger, Gastroenterol (1981); 80:962-969). Butyrate is thought to play a most critical role in the cell biology of colonocytes and is preferred over acetate and propionate as their oxidative fuel (Cummings, Gut (1981) 22:763-779). Butyrate inhibits the proliferation of malignant cells from the human colon in vitro via inhibition of DNA synthesis and arresting of the cells in the GI phase. Induction of cell differentiation has also been demonstrated, an observation that is consistent with the fact that when cells differentiate they lose their capacity to proliferate. Butyrate also enhances the capacity of colonic cells to repair DNA damage (Smith, Carcinogenesis (1986) 7:423-429). All of these effects require physical presence of the acid and are obtained at butyrate concentrations similar to those found in the colon in vivo. A particular point of interest is that there is evidence that human faecal inocula ferment starch to butyrate (Pilch (ed) Physiological effects and health consequences of dietary fibre. Bethesda Md. USA: FASEB 1987) and such production might explain inconsistencies in epidemiological data where fibre is not always protective but plant foods are beneficial.
Several studies in animal models have shown that supplementation of the diet with fibre protects against tumours induced with chemical carcinogens such as dimethylhydrazine (DMH), azoxymethane (AOM), and 3,2-dimethyl-4-aminobiphenyl (DMAB). Meta-analysis of these studies by the Federation of American Societies for Experimental Biology (FASEB) (Pilch (1987) supra) showed that wheat bran was more effective than pectin or cellulose in reducing lesion formation induced by chemical carcinogens. These data are paradoxical if one considers that soluble NSP might be expected to be fermented to SCFA more than wheat bran. However, rat studies show that wheat bran gives relatively higher concentrations of butyrate in hind-gut digesta than soluble NSP. In addition, wheat bran seems to bind chemical carcinogens and to reduce their colonic concentration and might be doing so in the animal model systems. A protective action of wheat against experimental carcinogenesis cannot be dismissed.
It is believed that butyrate enhances the proliferation of normal cells but may exert antineoplastic effects on susceptible cells and significantly retards the growth of human colon cancer cells in vitro (Kim et al In Malt and Williams (Eds) Colonic carcinogenesis. Lancaster MTP Press, (192); Falk Symposium 31: 317-323). A recent study which has shown that the molar proportion of butyrate is significantly lower in faeces from patients with adenomatous polyps (Weaner et al Gut (1988); 29: 1539-1543) is of special interest as it suggests that short chain fatty acid production is abnormal. In a feeding trial in patient with polyposis, a wheat bran supplement appeared to reduce polyp numbers and size (De Cosse et al J Nat Cancer Inst. (1989); 81:1290-1297). This is also a very promising study and indicates that insoluble NSP may be protective. Of particular interest is the fact in this study an insoluble NSP (which also enhances lactation) was protective. The situation with soluble NSP and resistant starch is unknown.
It is suggested that lack of luminal SCFAs lead in the short term to muscular atrophy and in the long term to ‘nutritional colitis’. This is especially evident in diversion colitis, which develops after complete diversion of the faecal stream and subsides after restoration of colorectal continuity. Irrigation with SCFA for 2-3 weeks has resulted in resolution of inflammation. Ulcerative colitis has also been successfully treated using butyrate enemas. (Scheppach et al (1992) Gasteroenterology; 103:51-56. Generally anti-inflammatory measures, such as the use of anti inflammatory drugs, do have side effects and in particular where large doses are used to overcome the natural degradation of those drugs in the small intestine before they reach the colon. The use of SCFA on the other hand is seen as particularly beneficial because they are naturally occuring and replace the use of anti-inflammatory drugs such as NSAIDS, corticosteroids and other anti inflammatory drugs.
General
Those skilled in the art will be aware that the invention described herein is subject to variations and modifications other than those specifically described. It is to be understood that the invention described herein includes all such variations and modifications. The invention also includes all such steps, features, compositions and compounds referred to or indicated in this specification, individually or collectively, and any and all combinations of any two or more of said steps or features.
Throughout this specification, unless the context requires otherwise the word “comprise”, and variations such as “comprises” and “comprising”, will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps. The present invention is not to be limited in scope by the specific embodiments described herein, which are intended for the purposes of exemplification only. Functionally-equivalent products, compositions and methods are clearly within the scope of the invention, as described herein.
Bibliographic details of the publications referred to by the inventors in this specification are collected at the end of the description. The references mentioned herein are hereby incorporated by reference in their entirety. Reference herein to prior art, including any one or more prior art documents, is not to be taken as an acknowledgment, or suggestion, that said prior art is common general knowledge in Australia or forms a part of the common general knowledge in Australia.
As used herein, the term “derived from” shall be taken to indicate that a particular integer or group of integers has originated from the species specified, but has not necessarily been obtained directly from the specified source.
The designation of nucleotide residues referred to herein are those recommended by the IUPAC-IUB Biochemical Nomenclature Commission, wherein A represents Adenine, C represents Cytosine, G represents Guanine, T represents Thymidine.